Butyl rubber (IIR), a random copolymer of isobutylene and isoprene is well known for its excellent thermal stability, ozone resistance and desirable dampening characteristics. IIR is prepared commercially in a slurry process using methyl chloride as a diluent and a Friedel-Crafts catalyst as the polymerization initiator. The methyl chloride offers the advantage that AlCl3, a relatively inexpensive Friedel-Crafts catalyst, is soluble in it, as are the isobutylene and isoprene comonomers. Additionally, the butyl rubber polymer is insoluble in the methyl chloride and precipitates out of solution as fine particles. The polymerization is generally carried out at temperatures of about −90° C. to −100° C. (see U.S. Pat. No. 2,356,128 and Ullmanns Encyclopedia of Industrial Chemistry, volume A 23, 1993, pages 288-295, the entire contents of each of which are herein incorporated by reference). The low polymerization temperatures are required in order to achieve molecular weights which are sufficiently high for rubber applications.
Recently there has been an emphasis on finding alternative diluents to the traditional chlorinated hydrocarbon, methyl chloride. Hydrofluorocarbons (HFC's) have similar properties to chlorinated hydrocarbons and are known refrigerants (see WO 2008/027518 and WO 2009/042847). Such HFC's, especially saturated HFC's, for example HFC-134a (1,1,1,2-tetrafluoroethane), have been identified as potential replacements for methyl chloride in polymerization processes involving higher temperatures (see U.S. Pat. No. 7,723,447, U.S. Pat. No. 7,582,715, U.S. Pat. No. 7,425,601, U.S. Pat. No. 7,423,100, U.S. Pat. No. 7,332,554, U.S. Pat. No. 7,232,872, U.S. Pat. No. 7,214,750, U.S. Pat. No. 7,699,962, US 2008/0290049, U.S. Pat. No. 7,781,547, U.S. Pat. No. 7,342,079, US 2007/0117939, US 2007/0299190, US 2007/0299161, US 2008/0234447, US 2008/0262180, U.S. Pat. No. 7,414,101, U.S. Pat. No. 7,402,636 and U.S. Pat. No. 7,557,170).
However, such saturated HFC's are strong greenhouse gases and their use is undesirable. The most studied HFC is HFC-134a (1,1,1,2-tetrafluroethane), also known as R134a, which has been broadly commercialized as a refrigerant in the 1990's to replace chlorofluorocarbons (CFC's) and hydrochlorofluorocarbons (HCFC's), which are ozone-depleting chemicals. The expanding use of HFC-134a is now posing a significant environmental threat as such HFC's are known to be powerful greenhouse gases. The GWP (Global-Warming Potential) of HFC-134a is 1430. There have been several discussions internationally to implement a controlled program to phase out of HFC-134a.
Further, cyclic oligomers are formed in significant quantities in butyl polymerization using either HFC-134a or methyl chloride as diluents or their blends thereof. These impurities are undesirable for pharmaceutical applications, such as rubber closures, due to the potential to extract the oligomers from the rubber. Furthermore, isoprenoid (short chain branching) structures are formed in significant quantities in butyl polymerization using methyl chloride as diluent. Isoprenoid structures limit the efficiency of subsequent halogenation reactions when producing halobutyl rubbers. Furthermore, when high isoprene butyl rubber is desired, traditional reactions require careful control of process conditions to increase isoprene levels in the butyl rubber.
Thus, there is still a need for polymerization vehicles that are relatively inexpensive, are not strong contributors to the greenhouse effect and/or provide improvement to the polymerization process. There is also still a need for butyl polymers having low levels of cyclic oligomers, low levels of isoprenoid structures and/or high levels of isoprene.